Levelshifters are used to translate digital signals from one level to another level. Level shifters are often used to shift core levels to I/O levels (e.g., from a 1v domain to a 3.3v domain). Levelshifters are typically used in I/Os and mixed signal cores. Virtually any integrated circuit (IC) that has more than one voltage supply domain (i.e., 1v, 1.2v, 2.5v, 3.3v, etc.) will use a levelshifter of some sort.
Referring to FIG. 1, a diagram of a circuit 10 is shown illustrating a conventional level shifter. The circuit 10 comprises a transistor pair Q1 and Q2, a transistor pair Q3 and Q4 and a transistor pair Q5 and Q6. The transistors Q1 and Q2 are shown implemented as thick oxide devices. The transistors Q3 and Q4 are shown implemented as thin oxide devices. The transistors Q5 and Q6 are shown implemented as thick oxide protection devices.
In one example, a supply voltage AVDD is 3.3v and an input signal IN and an input signal INZ (i.e., complementary digital signals) swing from 0v to 1v. When the signal IN is 1v, the signal INZ is 0v. Conversely when the signal IN is 0v, the signal INZ is 1v. A signal VBIAS is set with a voltage divider from the 3.3v supply at 1V+1 Vth, where Vth is the threshold voltage of the transistor Q5 (or the transistor Q6). Hence, the signal VBIAS is ˜1.7V. When the signal IN=1V and the signal and INZ=0V, then a signal OUT=3.3V and a signal OUTZ=0V. Conversely when the signal IN=0V and the signal INZ=1V, then the signal OUT=0V and the signal OUTZ=3.3V. When the signal IN changes dynamically from 0 to 1v, and the signal INZ changes from 1 to 0v, the signal OUTZ is pulled low from current flowing through both the transistor Q4 and the transistor Q5 while the transistor Q2 pulls the signal OUT to 3.3V and turns off the transistor Q1. Similarly, when the signal IN changes from 1 to 0v, and the signal INZ changes from 0 to 1v, the signal OUT is pulled low from current flowing through the transistors Q6 and Q3 while the transistor Q1 pulls the signal OUTZ to 3.3V and turns off the transistor Q2. In such cases, the swing on the drains of the transistors Q3 and Q4 is limited to 1v.
Conventional level shifters address either speed issues or reliability issues, but not both. Since reliability is normally a higher concern over speed, speed is sacrificed by lowering voltages across core devices to less than the core voltage.
It would be desirable to implement a level shifter that maximizes speed while limiting voltage stress on thin oxide devices within the level shifter to safe levels.